Multilayer films having at least five film layers, wherein at least one layer is flame retardant

ABSTRACT

The present invention provides unified multilayer films having at least one layer that includes a flame retardant film layer. In preferred embodiments the flame retardant film layer is an internal layer. In particularly preferred embodiments of the present invention, multilayer films include flame retardant layers alternating with non flame retardant layers. In other preferred embodiments, multilayer films include alternating layer of different flame retardant materials.

This application is a continuation-in-part of Ser. No. 08/980,920 filedon Dec. 1, 1997 now U.S. Pat. No. 6,045,895.

TECHNICAL FIELD

This invention relates to flame retardant films, and more particularly,multilayer films having flame-retardant layers.

BACKGROUND OF THE INVENTION

Flame retardant films have been used in many applications wherepolymeric properties offer unique performance advantages over propertiesof other inherently flame retardant materials such as metal sheets andfoils, and ceramics. Typically, the polymeric articles are eitherinherently flame retardant or rendered flame retardant by the additionof flame retardant additives. However, these approaches are limiting.

Polymer films made of inherently flame retardant polymers such aspolyvinyl chloride (PVC) and polyimide (PI) usually have a limited rangeof properties. For example plasticizers are generally added to PVC torender it more readily processable, however plasticizer migration oftenadversely affects adhesion to subsequent surfaces. In addition, PVCgenerally has little elasticity and low to moderate tensile strength.Similarly, PI is difficult to process and more expensive than mostcommon polymers.

Polymer films made of blends of polymer materials and flame retardantmaterials also have limited performance. While the range of polymermaterials is broad, the concentration of flame retardant material isgenerally high enough to significantly adversely affect mechanicalproperties of the polymer material. In addition, the flame retardantmaterials often migrate to the film surfaces and adversely affectadhesion to subsequent surfaces.

Thus, there is a need for new flame retardant polymeric films andarticles that have a broader range of mechanical mechanical propertiesand reduced surface fouling.

SUMMARY OF THE INVENTION

The present invention provides flame retardant films that not only havedesirable mechanical properties and reduced surface fouling, but haveimproved flame retardant efficiency and/or reduced cost when comparedwith conventional flame retardant polymer films or articles. The presentinvention provides unified multilayer films of at least five film layerswherein at least one layer, preferably an internal layer, comprises aflame retardant film layer and at least one layer comprises a non-flameretardant film layer.

Preferably, multilayer films include layers that include aflame-retardant film alternating with layers that include a film that isnot a flame retardant. In other preferred embodiments, multilayer filmshave layers of different flame retardant films. For example, theconstruction can include alternating layers of a first flame retardantfilm, a second flame retardant film and a non-flame retardant film.

One aspect of the present invention provides a multilayer film having aunified construction of at least 5, preferably 10, more preferably atleast 13 substantially contiguous film layers wherein at least one layer(preferably one internal layer) comprises a flame retardant film layerand at least one layer comprises a non-flame retardant film layer.

Another aspect of the present invention provides a multilayer filmhaving a unified construction; wherein the construction comprises atleast 5, preferably 10, more preferably at least 13 substantiallycontiguous layers of organic polymeric material; the constructioncomprising layers comprising a flame retardant film alternating withlayers comprising a film that is not flame retardant.

The present invention also provides a process of preparing aflame-retardant multilayer film. The process includes melt processingorganic polymeric material to form a unified construction of at least 5substantially contiguous film layers of organic polymeric material,wherein at least one internal layer of the organic polymeric materialcomprises a flame retardant film. Preferably, all the layers aresimultaneously melt processed, and more preferably, all the layers aresimultaneously coextruded.

A further aspect of the present invention provides a process ofpreparing a multilayer film, the process comprising melt processingorganic polymeric material to form a unified construction of at least 5substantially contiguous layers of organic polymeric material, theconstruction comprises film layers comprising a flame retardant filmlayer, alternating with non flame retardant film layers.

Herein, the following definitions are used:

“Unified” means that the layers are not designed to be separated ordelaminated as would a tape in roll form.

“Flame retardant” means a characteristic of basic flammability has beenreduced by some modification as measured by one of the accepted testmethods such as the Horizontal Burn or Hanging Strip tests.

“Flame retardant additive” means a compound or mixture of compounds thatwhen incorporated (either chemically or mechanically) into a polymerserves to slow or hinder the ignition or growth of fire.

“Flame retardant films” means polymeric films which are inherently flameretardant, or have been rendered flame retardant by means of a flameretardant additive.

“Melt processable” means polymers that are fluid or pumpable at thetemperatures used to melt process the films (e.g., about 50° C. to about300° C.), and do not significantly degrade or gel at the temperaturesemployed during melt processing.

“Pressure sensitive adhesive” means an adhesive that displays permanentand aggressive tackiness to a wide variety of substrates after applyingonly light pressure. It has a four-fold balance of adhesion, cohesion,stretchiness, and elasticity, and is normally tacky at use temperatures,which is typically room temperature (i.e., about 20° C. to about 30°C.).

“Melt viscosity” means the viscosity of molten material at theprocessing temperatures and shear rates employed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to multilayer products in the form offilms of organic polymeric material, wherein the films have at least onelayer, preferably at least one internal layer that includes a flameretardant film layer and at least one layer comprises a non-flameretardant film layer. Each of the other layers may include a flameretardant film layer, or a film layer that is not a flame retardant. Incertain preferred embodiments there are flame retardant film layersalternating with film layers that are not flame retardant. In otherpreferred embodiments there are alternating layers of a first flameretardant film layer, a second flame retardant film layer and anon-flame retardant film layer. The two outermost film layers may beflame retardant films, non-flame retardant films, or one of theoutermost layers may include a flame retardant film layer and the othera film layer that is non flame retardant. Each layer of the constructionis continuous and has a substantially contiguous relationship to theadjacent layers. Preferably, each layer is substantially uniform inthickness. The multiple layers in any one construction are “unified”into a single multilayer film such that the layers do not readilyseparate.

Flame retardant films used in the flame retardant film layer(s) includefilms which are inherently flame retardant, or have been rendered flameretardant by means of a flame retardant additive. Inherently flameretardant films are prepared from polymers, which due to their chemicalstructure either do not support combustion, or are self-extinguishing.These polymers often have increased stability at higher temperatures byincorporating stronger bonds (such as aromatic rings or inorganic bonds)in the backbone of the polymers or are highly halogenated. Examples ofinherently flame retardant polymers include poly(vinyl chloride),poly(vinylidine chloride), polyimides, polybenzimidazoles, polyetherketones, polyphosphazenes, and polycarbonates. Useful inherently flameretardant films generally have a Limiting Oxygen Index (LOI) of at least28% as determined by ASTM D-2863-91.

Useful flame retardant additives include halogenated organic compounds,organic phosphorus-containing compounds (such as organic phosphates),inorganic compounds and inherently flame retardant polymers. Theseadditives are added to or incorporated into the polymeric matrix of thepolymer film to render an otherwise flammable polymer flame retardant.The nature of the flame retardant additive is not critical and a singleadditive may be used. Optionally, it may be desirable to use a mixtureof two or more individual flame retardant additives.

Halogenated organic flame retardant additives are thought to function bychemical interaction with the flame: the additive dissociates intoradical species that compete with chain propagating and branching stepsin the combustion process. Useful halogenated additives are described,for example, in the Kirk-Othmer Encyclopedia of Technology, 4^(th) Ed.,vol. 10, pp 954-76, John Wiley & Sons, N.Y., N.Y., 1993.

Included within the scope of halogenated organic flame retardantadditives are substituted benzenes exemplified by tetrabromobenzene,hexachlorobenzene, hexabromobenzene, and biphenyls such as2,2′-dichlorobiphenyl, 2,4′-dibromobiphenyl, 2,4′-dichlorobiphenyl,hexabromobiphenyl, octabromobiphenyl, decabromobiphenyl and halogenateddiphenyl ethers, containing 2 to 10 halogen atoms.

The preferred halogenated organic flame retardant additives for thisinvention are aromatic and aliphatic halogen compounds such asbrominated benzene, brominated imides, chlorinated biphenyl, or acompound comprising two phenyl radicals separated by a divalent linkinggroup (such as a covlaent bond and having at least two chlorine orbromine atoms per phenyl nucleus, chlorine containing aromaticpolycarbonates, and mixtures of at least two of the foregoing.Especially preferred are hexabromobenzene, decabromodiphenyl oxide andtetrabromobisphenol A.

Among the useful organic phosphorus additives are organic phosphoruscompounds, phophorus-nitrogen compounds and halogenated organicphosphorus compounds. Often organic phosphorus compounds function asflame retardants by forming protective liquid or char barriers, whichminimize transpiration of polymer degradation products to the flameand/or act as an insulating barrier to minimize heat transfer.

In general, the preferred phosphate compounds are selected from organicphosphonic acids, phosphonates, phosphinates, phosphonites,phosphinites, phosphine oxides, phosphines, phosphites or phosphates.Illustrative is triphenyl phosphine oxide. These can be used alone ormixed with hexabromobenzene or a chlorinated biphenyl and, optionally,antimony oxide. Phosphorus-containing flame retardant additives aredecribed, for example, in Kirk-Othmer (supra) pp. 976-98.

Typical of the preferred phosphorus compounds to be employed in thisinvention would be those having the general formula

and nitrogen analogs thereof where each Q represents the same ordifferent radicals including hydrocarbon radicals such as alkyl,cycloalkyl, aryl, alkyl substituted aryl and aryl substituted alkyl;halogen, hydrogen and combinations thereof provided that at least one ofsaid Q's is aryl. Typical examples of suitable phosphates include,phenylbisdodecyl phosphate, phenylbisneopentyl phosphate, phenylethylenehydrogen phosphate, phenyl-bis-3,5,5′-trimethylhexyl phosphate),ethyidiphenyl phosphate, 2-ethylhexyl di(p-tolyl) phosphate, diphenylhydrogen phosphate, bis(2-ethyl-hexyl) p-tolylphosphate, tritolylphosphate, bis(2-ethylhexyl)-phenyl phosphate, tri(nonylphenyl)phosphate, phenylmethyl hydrogen phosphate, di(dodecyl) p-tolylphosphate, tricresyl phosphate, triphenyl phosphate, halogenatedtriphenyl phosphate, dibutylphenyl phosphate, 2-chloroethyldiphenylphsophate, p-tolyl bis(2,5,5′-trimethylhexyl) phosphate,2-ethylhexyldiphenyl phosphate, diphenyl hydrogen phosphate, and thelike. The preferred phosphates are those where each Q is aryl. The mostpreferred phosphate is triphenyl phosphate. It is also preferred to usetriphenyl phosphate in combination with hexabromobenzene and,optionally, antimony oxide.

Also suitable as flame-retardant additives for this invention arecompounds containing phosphorus-nitrogen bonds, such as phosphonitrilicchloride, phosphorus ester amides, phosphoric acid amides, phosphonicacid amides or phosphinic acid amides.

Among the useful inorganic flame retardant additives include compoundsof antimony, such as antimony trioxide, antimony pentoxide, and sodiumantimonate; boron, such as barium metaborate, boric acid, sodium borateand zinc borate; aluminum, such as aluminum trihydroxide, magnesium,such as magnesium hydroxide; molybdenum, such as molybdic oxide,ammonium molybdate and zinc molybdate, phosphorus, such as phosphoricacid; and tin, such as zinc stannate. The mode of action is often variedand may include inert gas dilution, (by liberating water for example),and thermal quenching (by endothermic degradation of the additive).Useful inorganic additives are described for example in Kirk-Othmer(supra), pp 936-54.

Especially useful are mixed additives of an antimony additive and ahalogenated organic additive, describes as “antimony-halogen” additiveswhich produces an especially effective flame retardant. The twoadditives react synergistically at flame temperatures to produce anantimony halide or oxyhalide which produce radical species (whichcompete with chain propagating and branching steps in the combustionprocess) as well as promoting char formation.

Inherently flame retardant polymers may be used in the form of films, inthe form of particles dispersed in a polymer matrix, or as a blend in acompatible polymer. Examples of inherently flame retardant polymersinclude poly(vinyl chloride), poly(vinylidine chloride), polyimides,polybenzimidazoles, polyether ketones, polyphosphazenes, polycarbonatesand polysiloxanes.

The additives are generally incorporated into the flame retardant filmlayers by addition of the additive(s) to the melt prior to filmformation. The materials may be added neat, as a melt blend of theadditive in a polymer, or with the use of a cosolvent or comptatibilizerto render the additive and polymer compatible, which may be subsequentlyremoved prior to film formation. When using an inherentlyflame-retardant polymer as an additive, it may be melt blended ifcompatible, or a cosolvent or compatibilizer may be used. Alternativelythe inherently flame-retardant polymer additive may be added as fineparticles to the melt. In the case of halogenated organic additives andorganic phosphorus additives, they may be added neat in the form ofliquid or solids to the melt. Care should be exercised to choose anadditive that is stable at the melt temperature of the polymer.

The particle size of the inorganic additive (or organic additives whichdo not melt) should be less than the the thickness of the flameretardant-film layer(s) into which it is incorporated to ensure uniformthickness of the multilayer film. Preferably the particle size is lessthan one-half, more preferably less than one-third the thickness of theflame retardant film layer(s). In general, the smaller the particle, orthe more surface area the particle presents, the more effective theflame retardant properties.

Generally, when using a low viscosity liquid flame retardant additive,it is preferable to use a low viscosity polymer, whereby the bestdispersion is obtained when the two viscosities of the polymer matrixand dispersed phases are closely matched. Alternatively, when using asolid flame retardant it is preferable to use a high viscosity polymeras high viscosities are necessary to generate the stresses necessary toproduce a homogenous dispersion. Viscosities may also be matched byjudicious selection of process temperature conditions. Furtherinformation on multiphase flow in polymer processing may be found inHan, Multiphase Flow in Polymer Processing, Academic Press, N.Y., 1981,pp 229-235 and in Elmendorpp, Dispersive Mixing in Liquid Systems,Mixing in Polymer Processing, C. Rauwendaal, ed., Marcel Dekker, Inc.,N.Y., pp. 17-53.

Flame retardant additives are added in sufficient amounts to render themultilayer film flame retardant. Generally the additives are added inamounts of 40 wt. % or more in each flame retardant layer. Preferablythe additives are added in amounts of at least 50 wt. % in each flameretardant layer or in the amounts of 10 to 90 wt. % of the unifiedmultilayer film.

Polymeric materials used in the multilayer films of the presentinvention include one or more melt-processible organic polymers, whichmay include thermoplastic, thermoplastic elastomeric or elastomericmaterials. Thermoplastic materials are generally materials that flowwhen heated sufficiently above their glass transition temperature, or ifsemicrystalline, above their melt temperatures, and become solid whencooled. They may be elastomeric or nonelastomeric.

Thermoplastic materials useful in the present invention that aregenerally considered nonelastomeric include, for example, polyolefinssuch as isotactic polypropylene, low density polyethylene, linear lowdensity polyethylene, very low density polyethylene, medium densitypolyethylene, high density polyethylene, polybutylene, nonelastomericpolyolefin copolymers or terpolymers such as ethylene/propylenecopolymer and blends thereof; ethylene-vinyl acetate copolymers such asthose available under the trade designation ELVAX from E.I. DuPont deNemours, Inc., Wilimington, Del.; ethylene acrylic acid copolymers;ethylene methacrylic acid copolymers such as those available under thetrade designation SURLYN from E.I. DuPont de Nemours, Inc.;polymethylmethacrylate; polystyrene; ethylene vinyl alcohol; polyestersincluding amorphous polyester; and polyamides.

Elastomers, as used herein, are distinct from thermoplastic elastomericmaterials in that the elastomers require crosslinking via chemicalreaction or irradiation to provide a crosslinked network which impartsmodulus, tensile strength, and elastic recovery. Elastomers useful inthe present invention include, for example, natural rubbers such asCV-60, a controlled viscosity grade of rubber, and SMR-5, a ribbedsmoked sheet rubber; butyl rubbers, such as Exxon Butyl 268 availablefrom Exxon Chemical Co., Houston, Tex.; synthetic polyisoprenes such asCARIFLEX, available from Shell Oil Co., Houston, Tex., and NATSYN,available from Goodyear Tire and Rubber Co., Akron, Ohio;ethylene-propylenes; polybutadienes; polybutylenes; polyisobutylenessuch as VISTANEX, available from Exxon Chemical Co.; andstyrene-butadiene random copolymer rubbers such as AMERIPOL SYNPOLavailable from American Synpol Co., Port Neches, Tex.

In the present invention, preferred organic polymers and homo-andcopolymers of polyolefins including polyethylene, polypropylene andpolybutylene homo- and copolymers.

Thermoplastic materials that have elastomeric properties are typicallycalled thermoplastic elastomeric materials. Thermoplastic elastomericmaterials are generally defined as materials that act as though theywere covalently crosslinked at ambient temperatures, exhibiting highresilience and low creep, yet process like thermoplastic nonelastomersand flow when heated above their softening point. Thermoplasticelastomeric materials useful in the multilayer films of the presentinvention include, for example, linear, radial, star, and tapered blockcopolymers (e.g., styrene-isoprene block copolymers,styrene-(ethylene-butylene) block copolymers,styrene-(ethylene-propylene) block copolymers, and styrene-butadieneblock copolymers); polyetheresters such as that available under thetrade designation HYTREL from E.I. DuPont de Nemours, Inc.; elastomericethylene-propylene copolymers; thermoplastic elastomeric polyurethanessuch as that available under the trade designation MORTHANE URETHENEfrom Morton International, Inc., Chicago, Ill.; polyvinylethers;poly-α-olefin-based thermoplastic elastomeric materials such as thoserepresented by the formula —(CH₂CHR)_(x) where R is an alkyl groupcontaining 2 to 10 carbon atoms, and poly-α-olefins based on metallocenecatalysis such as ENGAGE, ethylene/poly-α-olefin copolymer availablefrom Dow Plastics Co., Midland, Mich.

The multilayer films are typically prepared by melt processing (e.g.,extruding). In a preferred method, the flame retardant and non-flameretardant layers are generally formed at the same time, joined while ina molten state, and cooled. That is, preferably, the layers aresubstantially simultaneously melt-processed, and more preferably, thelayers are substantially simultaneously coextruded. Products formed inthis way possess a unified construction and have a wide variety ofuseful, unique, and unexpected properties, which provide for a widevariety of useful, unique, and unexpected applications.

Preferably, the multilayer films range in thickness from about 25 toabout 750 micrometers (μm) thick (more preferably, no greater than about150 μm, and most preferably, no greater than about 50 μm). The thickness(or volume fraction) of the multilayer film and the individual filmlayers depend primarily on the end-use application and the desiredcomposite mechanical properties of the multi-layered film. Suchmultilayer films have a construction of at least 5 layers, preferably,at least 10 layers, more preferably, at least 13 layers, and even morepreferably, at least 29 layers. For preferred embodiments, there aregenerally no more than about 500 layers, more preferably, no more thanabout 200 layers, and most preferably, no more than about 100 layers,although it is envisioned that constructions having many more layers canbe made using the materials and methods described herein.

Depending on the polymers and additives chosen, thicknesses of thelayers, and processing parameters used, the multilayer films willtypically have different properties at different numbers of layers. Thatis, the same property (e.g., tensile strength, modulus, fire retardancy)may go through maximum at a different number of layers for twoparticular materials when compared to two other materials.

In any one construction of the alternating layers of flame retardantfilm layers and non flame retardant film layers, each of the flameretardant layers typically includes the same material (flame retardantadditive in a polymer matrix or in an inherently flame retardantpolymer) or combination of materials, although they may includedifferent materials or combinations of materials. Similarly, each of thelayers that is not flame retardant typically includes the same materialor combination of materials, although they may include differentmaterials or combinations of materials.

Multilayer films can include an (AB)_(n) construction, with either Aand/or B layers as the outermost layers (e.g., (AB)_(n)A, (BA)_(n)B, or(AB)_(n)). In such constructions, each of the B layers has flameretardant properties as a result of the incorporation of a flameretardant additive or the use of an inherently flame retardant polymer,which may be the same or different in each layer, and each of the Alayers does not have flame retardant properties, which may be the sameor different in each layer. Multilayer films can also include A, A′ B,and B′ film layers, with any of the A, A′, B or B′ layers as theoutermost layers. Preferably the A layers are the outermost layers. Insuch constructions, each of the B and B′ layers may include a differentflame retardant film layer and each of the B layers may include adifferent non-flame retardant film layer. In each of theseconstructions, n is preferably at least 2, and more preferably, at least5, depending on the materials used and the application desired.

In embodiments with alternating different flame retardant layers (B,B′),the multilayer films can take advantage of the properties of each of theflame retardant film layers. For example, a construction withalternating layers of an organic halogenated flame retardant and aninorganic antimony trioxide flame retardant has the synergistic effectof reducing the concentration of radical species and promoting charformation. Similarly the use of organic halogenated flame retardant andhydrated alumina will retard flames by reducing radical species and theenthalpy of combustion.

Preferred embodiments include three or more layers of a flame-retardantadditive in a polymer matrix and three or more layers of the samepolymer matrix that is not a flame retardant (i.e lacking theflame-retardant additive). More preferred embodiments include only twotypes of materials, one inherently flame retardant polymer and one thatis not flame retardant in alternating layers. Other preferredembodiments include only two different flame-retardants in alternatinglayers.

The two outermost layers of multilayer films of the present inventioncan include one or more flame-retardant films, which may be the same ordifferent in each of the two outermost layers. Alternatively, the twooutermost layers can include one or more films that are not flameretardant, which may be the same or different in each of the twooutermost layers. Furthermore, the inventive films include embodimentsin which only one of the outermost layers includes one or moreflame-retardant films.

The individual layers of multilayer films of the present invention canbe of the same or different thicknesses. Preferably, each internal layeris no greater than about 25 micrometers (μm) thick, and more preferably,no greater than about 5 μm thick. Each of the two outermost layers canbe significantly thicker than any of the inner layers, however.Preferably, each of the two outermost layers is no greater than about150 μm thick, more preferably no greater than 50 μm thick. Typically,each layer, whether it be an internal layer or one of the outermostlayers, is at least about 0.01 μm thick, depending upon the materialsused to from the layer and the desired application.

Multilayer films wherein one or more of the layers is a flame retardantcan be made that have many significant and unexpected properties. Thesecan include, for example, good flame resistance, reduced surfacefouling, good weatherability, relatively low material costs, good flameresistance, and sufficient tensile strength for handling, relativelyhigh break elongation and toughness, relatively high yield and breakstress, good drape and softness, good stretch release properties, andpaper-like tensile, elongation and tear properties. Each multilayer filmof the present invention will not necessarily have all of theseadvantageous properties. This will depend on the number of layers, thetypes of materials, the affinity of the materials for each other, themodulus of the different materials, and the like.

Preferably one or both of the outer layers are not flame retardant, themultilayer films can be used as single- or double-sided flame retardanttapes, nonadhesive films for use as backings for tapes, or flameretardant films for use as adhesive layers in tapes, for example. Thisis because they have advantageous mechanical properties, tensilestrength, a relatively high break elongation (i.e., elongation at break)and toughness, good yield and break stress, as well as beneficial tearproperties, despite the incorporation of one or more flame retardantfilm layers.

When used as a backing for an adhesive tape, the multilayer film of thepresent invention may further comprise a pressure-sensitive adhesivelayer. Pressure sensitive adhesives useful in the present invention canbe self tacky or require the addition of a tackifier. Such materialsinclude, but are not limited to, tackified natural rubbers, tackifiedsynthetic rubbers, tackified styrene block copolymers, self-tacky ortackified acrylate or methacrylate copolymers, self-tacky or tackifiedpoly-α-olefins, and tackified silicones. Examples of suitable adhesivesare described in U.S. Pat. No. Re 24,906 (Ulrich), U.S. Pat. No.4,833,179 (Young et al.), U.S. Pat. No. 5,209,971 (Babu et al.), U.S.Pat. No. 2,736,721 (Dexter), and U.S. Pat. No. 5,461,134 (Leir et al.),for example. Others are described in the Encyclopedia of Polymer Scienceand Engineering, vol. 13, Wiley-lnterscience Publishers, New York, 1988,the Encyclopedia of Polymer Science and Technology, vol. 1, lntersciencePublishers, New York, 1964 and in D. Satas, Handbook of PressureSensitive Adhesives, 2^(nd) Edition, Van Nostrand Reinhold, New York,1989.

A pressure sensitive adhesive useful in the present invention typicallyhas an open time tack (i.e., period of time during which the adhesive istacky at room temperature) on the order of days and often months oryears. An accepted quantitative description of a pressure sensitiveadhesive is given by the Dahlquist criterion line (as described inHandbook of Pressure Sensitive Adhesive Technology, Second Edition, D.Satas, ed., Van Nostrand Reinhold, New York, N.Y., 1989, pages 171-176),which indicates that materials having a storage modulus (G′) of lessthan about 3×10⁵ Pascals (measured at 10 radians/second at a temperatureof about 20° C. to about 22° C.) typically have pressure sensitiveadhesive properties while materials having a G′ in excess of this valuetypically do not.

Suitable polymers for use in preparing the films of the presentinvention, whether they are inherently flame retardants or not, are meltprocessable. That is, they are fluid or pumpable at the temperaturesused to melt process the films (e.g., about 50° C. to about 300° C.),and they are film formers. Furthermore, suitable polymers do notsignificantly degrade or gel at the temperatures employed during meltprocessing (e.g., extruding or compounding). Preferably, such polymershave a melt viscosity of about 10 poise to about 1,000,000 poise, asmeasured by capillary melt rheometry at the processing temperatures andshear rates employed in extrusion. Typically, suitable polymers possessa melt viscosity within this range at a temperature of about 175° C. anda shear rate of about 100 seconds⁻¹.

In melt processing multilayer films of the present invention, thepolymers in adjacent layers need not be chemically or physicallycompatible or well matched, particularly with respect to meltviscosities, although they can be if so desired. That is, althoughmaterials in adjacent polymeric flowstreams can have relative meltviscosities (i.e., ratio of their viscosities) within a range of about1:1 to about 1:2, they do not need to have such closely matched meltviscosities. Rather, the materials in adjacent polymeric fiowstreams canhave relative melt viscosities of at least about 1:5, and often up toabout 1:20. For example, the melt viscosity of a flowstream of polymer B(or A) can be similar or at least about 5 times, and even up to about 20times, greater than the melt viscosity of an adjacent flowstream ofpolymer A (or B).

In melt processing polymers of different flame retardants film layersand/or non flame retardant film layers, the differences in elasticstresses generated at the interface between the layers of differentflame retardants is also important. Preferably, these elasticdifferences are minimized to reduce or eliminate flow instabilities thatcan lead to layer breakup. With knowledge of a material's elasticity, asmeasured by the storage modulus on a rotational rheometer over a rangeof frequencies (0.001 rad/sec. <ω<100 rad/sec.) at the processingtemperature, along with its viscosity at shear rates less than 0.01second⁻¹, and the degree to which the material's viscosity decreaseswith shear rate, one of skill in the art can make judicious choices ofthe relative thicknesses of the layers, the die gap, and the flow rateto obtain a film with continuous, uniform layers. Generally, the elasticstresses at 100 sec⁻¹ by a more viscous polymer should be greater thanthe elastic stress generated by the less viscous polymer. Further, theratio of the storage modulus to the viscosity at 0.01 sec⁻¹ for the moreviscous polymer should be greater than that of the less viscous polymer.

Significantly, relatively incompatible materials (i.e., those thattypically readily delaminate as in conventional two layer constructions)can be used in the multilayer films of the present invention. Althoughthey may not be suitable for all constructions, they are suitable forthe constructions having larger numbers of layers. That is, generally asthe number of layers increases, relatively incompatible materials can beused without delamination occurring. in addition, film properties suchas elongation at break and toughness often increase as the number oflayers increases, depending on the materials used.

The flame retardant layer (B or B′) can include a single flameretardant, a mixture (e.g., blend) of several flame retardants, or amixture (e.g., blend) of a flame retardant and a material that is not aflame retardant (e.g., a nontacky thermoplastic material, which may ormay not be elastomeric), as long as the layer has flame retardantproperties. Examples of some flame retardant blends are described inKirk-Othmer (supra). Similarly, the nonflame retardant layer (A or A′)can include a single polymer that is not a flame retardant, a mixture ofseveral such polymers, , as long as the layer does not have flameretardant properties.

The materials of the non-flame retardant layer (A or A′) can be modifiedwith one or more processing aids, such as plasticizers and lubricants,to modify their properties. Plasticizers and lubricants useful with thepolymeric materials are preferably miscible at the molecular level,i.e., dispersible or soluble in the thermoplastic material. Externallubricants that are incompatible with the polymer can also be added thatact by migrating to the surface of the polymer melt and reducingfrictions with the extrusion equipment (the die or extruder barrel forexample). Examples of plasticizers and lubricants include, but are notlimited to, polybutene, paraffinic oils and waxes, fatty acids includingstearic acid and calcium stearate, petrolatum, liquid rubbers, andcertain phthalates with long aliphatic side chains such as ditridecylphthalate. When used, a processing aid is typically present in an amountof about 5 parts to about 300 parts by weight, and preferably up toabout 200 parts by weight, based on 100 parts by weight of the polymericmaterial in the nonflame retardant layer.

Other additives, such as fillers, pigments, crosslinking agents,antioxidants, ultraviolet stabilizers, and the like, may be added tomodify the properties of either the flame retardant layers (B or B) orthe nonflame retardant layers (A or A′). Each of these additives is usedin an amount to produce the desired result.

Pigments and fillers can be used to modify cohesive strength andstiffness, cold flow, and tack, as well as chemical resistance and gaspermeability. For example, aluminum hydrate, lithopone, whiting, and thecoarser carbon blacks such as thermal blacks also increase tack withmoderate increase in cohesivity, whereas clays, hydrated silicas,calcium silicates, silico-aluminates, and the fine furnace and thermalblacks increase cohesive strength and stiffness. Platy pigments andfillers, such as mica, graphite, and talc, are preferred for acid andchemical resistance and low gas permeability. Other fillers can includeglass or polymeric beads or bubbles, metal particles, fibers, and thelike. Typically, pigments and fillers are used in amounts of about 0.1%to about 20% by weight, based on the total weight of the multilayerfilm.

Crosslinkers such as benzophenone, derivatives of benzophenone, andsubstituted benzophenones such as acryloyloxybenzophenone may also beadded. Such crosslinkers are preferably not thermally activated, but areactivated by a source of radiation such as ultraviolet or electron-beamradiation subsequent to forming the films. Typically, crosslinkers areused in amounts of about 0.1% to about 5.0% by weight, based on thetotal weight of the multilayer film.

Antioxidants and/or ultraviolet stabilizers may be used to protectagainst severe environmental aging caused by ultraviolet light or heat.These include, for example, hindered phenols, amines, and sulfur andphosphorus hydroxide decomposers. Typically, antioxidants and/orultraviolet stabilizes are used in amounts of about 0.1% to about 5.0%by weight, based on the total weight of the multilayer film.

Intermediate layers may be used in a multilayered construction to adheredifferent polymeric materials having insufficient interlayer adhesion.Intermediate layers, or tie layers, generally have an affinity for bothof the principal layers and typically consist of materials that will notsignificantly reduce the overall tensile properties of the multilayerconstruction. Some useful tie layers include, for example, copolymerscontaining blocks that have an affinity for each of the principallayers, which flow when melted and cool to a tack-free state.

Tie layers, which are typically hot melt adhesive (i.e., tacky when inthe melt state), can also be used to enhance the adhesion between eachof the layers if so desired. Materials useful in the tie layers include,ethylene/vinyl acetate copolymer (preferably containing at least about10% by weight of vinyl acetate units), carboxylated ethylene/vinylacetate copolymer such as that available under the trade designationCXA, from E.I. DuPont de Nemours, Inc., copolymers of ethylene andmethyl acrylate such as that commercially available under the tradedesignation POLY-ETH EMA, from Gulf Oil and Chemicals Co.,ethylene/acrylic acid copolymer such as that available under the tradedesignation SURLYN (a copolymer of ethylene with a methacryic acid) fromE.I. DuPont de Nemours, Inc., maleic anhydride modified polyolefins andcopolymers of polyolefins such as that commercially available under thetrade designation MODIC, from Mitsubishi Chemical Co., polyolefinscontaining homogeneously dispersed vinyl polymers such as thosecommercially available under the trade designation VMX from MitsubishiChemical Co. (e.g., FN-70, an ethylene/vinyl acetate based producthaving a total vinyl acetate content of 50% and JN-70, an ethylene/vinylacetate based product containing dispersed polymethylmethacrylate andhaving a vinyl acetate content of 23% and a methyl methacrylate contentof 23%), POLYBOND (believed to be a polyolefin grafted with acrylicacid) from B.P. Chemicals Inc., Cleveland, Ohio, PLEXAR (believed to bea polyolefin grafted with ftinctional groups) from Quantum Chemicals,inc., Cincinnati, Ohio, a copolymer of ethylene and acrylic acid such asthat commercially available under the trade designation PRIMACOR fromDow Chemical Co., Midland, Mich., and a copolymer of ethylene andmethacrylic acid such as that commercially available under the tradedesignation NUCREL from E.I. DuPont de Nemours, Inc.

The multilayer films of the present invention can be prepared directlyby extrusion, for example, with the outermost layers being preferablynon flame retardant. Frequently, incorporation of a flame retardant intoone or both of the outermost layers can degrade the surface and/ormechanical properties of the outermost layer. Halogenated organic flameretardants, for example, may tend to migrate to the surface of the filmand render the surface non-amenable to further coating, by a pressuresensitive adhesive for example. Alternatively, the films can be madewith one or both of the outermost layers being flame retardant layer(s)depending on the application.

The multilayer films of the present invention can be used as thebackings or substrates for single-sided or double-sided adhesiveproducts, such as tapes. Preferably the multilayer films used asbackings in tape have a non flame retardant layer as at least one of theoutermost layers. Such films can be prepared using extrusion techniques,for example, to produce such products directly (i.e., with one or bothoutermost layers of the film being an a pressure sensitive adhesivelayer). Alternatively, a multilayer film can be coated with an adhesivematerial using conventional coating techniques. Furthermore, suchproducts can be coated with a low-adhesion backsize (LAB) material,which restricts adhesion of various types of surfaces to the film whenit is wound as a coil or is stacked on itself A wide variety of knownadhesive materials (e.g., any of the pressure sensitive materialsdescribed herein) and LAB materials (e.g., polyolefins, urethanes, curedsilicones, fluorochemicals) can be used as well as a wide variety ofknown coating techniques, including solvent coating and extrusioncoating techniques.

Multilayer films of the present invention can be made using a variety ofequipment and a number of melt-processing techniques (typically,extrusion techniques) well known in the art. Such equipment andtechniques are disclosed, for example, in U.S. Pat. No. 3,565,985(Schrenk et al.), U.S. Pat. No. 5,427,842 (Bland et al.), U.S. Pat.No.5,589,122 (Leonard et al.), U.S. Pat. No. 5,599,602 (Leonard et al.),and U.S. Pat. No. 5,660,922 (Herrid(e et al.). For example, single- ormulti-manifold dies, full moon feedblocks (such as those described inU.S. Pat. No. 5,389,324 to Lewis et al.), or other types of meltprocessing equipment can be used, depending on the number of layersdesired and the types of materials extruded.

For example, one technique for manufacturing multilayer films of thepresent invention can use a coextrusion technique, such as thatdescribed in U.S. Pat. No. 5,660,922 (Herridge et al.). In a coextrusiontechnique, various molten streams are transported to an extrusion dieoutlet and joined together in proximity of the outlet. Extruders are ineffect the “pumps” for delivery of the molten streams to the extrusiondie. The precise extruder is generally not critical to the process. Anumber of useful extruders are known and include single and twin screwextruders, batch-off extruders, and the like. Conventional extruders arecommercially available from a variety of vendors such as Davis-StandardExtruders, Inc. (Pawcatuck, Conn.), Black Clawson Co. (Fulton, N.Y.),Berstorff Corp. (N.C.), Farrel Corp. (Conn.), and Moriyama Mfr. Works,Ltd. (Osaka, Japan).

Other pumps may also be used to deliver the molten streams to theextrusion die. They include drum loaders, bulk melters, gear pumps, andthe like, and are commercially available from a variety of vendors suchas Graco LTI (Monterey, Calif.), Nordson (Westlake, Calif.), IndustrialMachine Manufacturing (Richmond, V.A.), and Zenith Pumps Div., ParkerHannifin Corp. (N.C.).

Typically, a feedblock combines the molten streams into a single flowchannel. The distinct layers of each material are maintained because ofthe laminar flow characteristics of the streams. The molten structurethen passes through an extrusion die, where the molten stream is reducedin height and increased in width so as to provide a relatively thin andwide construction. This type of coextrusion is typically used tomanufacture multilayer film constructions having about 10 layers ormore.

However, the use of a feedblock is optional, as a variety of coextrusiondie systems are known. For example, multimanifold dies may also beemployed, such as those commercially available from The Cloeren Company(Orange, Tex). In multimanifold dies, each material flows in its ownmanifold to the point of confluence. In contrast, when feedblocks areused, the materials flow in contact through a single manifold after thepoint of confluence. In multimanifold die manufacturing, separatestreams of material in a flowable state are each split into apredetermined number of smaller or sub-streams. These smaller streamsare then combined in a predetermined pattern of layers to form an arrayof layers of these materials in a flowable state. The layers are inintimate contact with adjacent layers in the array. This array generallycomprises a stack of layers which is then compressed to reduce itsheight. In the multimanifold die approach, the film width remainsconstant during compression of the stack, while the width is expanded inthe feedblock approach. In either case, a comparatively thin, wide filmresults. Layer multipliers in which the resulting film is split into aplurality of individual subfilms which are then stacked one upon anotherto increase the number of layers in the ultimate film may also be used.The multimanifold die approach is typically used in manufacturingmultilayer film constructions having less than about 10 layers.

In manufacturing the films, the materials may be fed such that either aflame retardant layer or the non-flame retardant layer forms theoutermost layers. The two outermost layers are often formed from thesame material. Preferably, although not necessarily, the materialscomprising the various layers are processable at the same temperature.Significantly, although it has been generally believed that the meltviscosities of the various layers should be similar, this is not anecessary requirement of the methods and products of the presentinvention. Accordingly, residence times and processing temperatures mayhave to be adjusted independently (i.e., for each type of material)depending on the characteristics of the materials of each layer

The volume fraction of the A and B layers depends primarily on the ratioof the viscosities of the component polymers or polymer mixtures(including the addition of the flame retardant additive). For example,if the outer “A” layer has a higher viscosity than the “B” layer,process stability considerations suggest that the “B” layer have agreater volume fraction (i.e >50%). Conversely, if the A layer has alower viscosity than the B layer, process stability should increase ifthe B layer has a smaller (i.e. <50%) volume fraction. Theseconsiderations are generally true regardless of the number of layers andthe total flow rate of the process.

Other manufacturing techniques, such as lamination, coating, orextrusion coating may be used in assembling multilayer films andproducts from such multilayer films according to the present invention.For example, in lamination, the various layers of the film are broughttogether under temperatures and/or pressures (e.g., using heatedlaminating rollers or a heated press) sufficient to adhere adjacentlayers to each other.

In extrusion coating, a first layer is extruded onto a cast web, auniaxially oriented film, or a biaxially oriented film, and subsequentlayers are sequentially coated onto the previously provided layers.Extrusion coating may be preferred over the melt coextrusion processdescribed above if it is desirable to pretreat selected layers of themultilayer film or if the materials are not readily coextrudable.

Continuous forming methods include drawing the multilayer film out of afilm die and subsequently contacting the extruded multilayer film with amoving plastic web or other suitable substrate, After forming, themultilayer films are solidified by quenching using both direct methods,such as chill rolls or water baths, and indirect methods, such as air orgas impingement.

The films of the present invention can be oriented, either uniaxially(i.e., substantially in one direction) or biaxially (i.e., substantiallyin two directions), if so desired. Such orientation can result inimproved strength properties, as evidenced by higher modulus and tensilestrength. Preferably, the films are prepared by co-extruding theindividual polymers to form a multi layer film and then orienting thefilm by stretching at a selected temperature. For example, uniaxialorientation can be accomplished by stretching a multilayer filmconstruction in the machine direction at a temperature of about themelting point of the film, whereas biaxial orientation can beaccomplished by stretching a multilayer film construction in the machinedirection and the cross direction at a temperature of about the meltingpoint of the film. Optionally heat-setting at a selected temperature mayfollow the orienting step.

EXAMPLES

This invention is further illustrated by the following examples whichare not intended to limit the scope of the invention. In the examples,all parts, ratios and percentages are by weight unless otherwiseindicated. The following test methods were used to characterize theflame retardant films in the examples:

Test Methods

Horizontal Burn

Burning characteristics of multilayer films were evaluated according toASTM D1000 except the film were first laminated to a 25 micrometer thicklayer of pressure-sensitive adhesive (a blend of 50 parts KRATON™ 1107polystyrene/poluisoprene block coplolymer available from Shell Chemical,50 parts NATSYNT™ 2210 polyisoprene homopolymer available from GoodyearTire and Rubber, 75 parts WINTACK PLUS™ hydrocarbon tackifier availablefrom Goodyear, 30 parts ENDEX 160™ end-block reinforcing resin and 2parts IRGANOX™ 1010 antioxidant available from CIBA-Giegy, as describedin U.S. Pat. No. 5,500,293) as in the vertical burn test) to permit thefilm to stick to a brass rod that was used in the test. The brass rodwas wrapped with two overlapping layers of tape and supported in ahorizontal position. A gas burner flame was applied for 30 seconds andimmediately removed. The time required for the sample to self-extinguishis measured. This test differentiates among tapes with wide ranges ofburning characteristics but is less precise for tapes of narrow rangesof burning characteristics.

Hanging Strip

Burning characteristics of multilayer films were evaluated according toASTM 568-77 A45 cm×25 cm with a 38 cm gauge length sample was suspendedfrom a clamp inside a protective metal shield that was located in a fumehood. A gas burner flame of a given height was applied until filmignited. Flame was removed immediately and the time needed to burn 38 cmof sample length or for sample to self-extinguish was measured. Thistest differentiates among tapes with wide ranges of burningcharacteristics but is less precise for tapes of narrow ranges ofburning characteristics.

Tensile Testing

Tensile properties of the multilayer films were evaluated using astandard tensile/elongation method on an Instron mechanical testingframe at 12 inches/minute (30.5 cm/minute). Sample were of 0.5 incheswidth (1.27 cm) and gauge length of 4 inches (10.2 cm). Thickness ofthesamples depended on process conditions and were measured using aMitutoyo Liner Thickness Gage.

Materials Used Material Description Fina ™ 3374 Isotactic polypropyleneavailable from Fina Oil & Chem, Dallas, TX. Rexflex ™ Significantlyatactic polypropylene available from W101 Huntsman Polypropylene Corp.,Woodbury, NJ. 1 Nat-2P-W A brominated imide and antimony trioxideblended into a polyethylene polymer at a 45:55 weight concentration witha 3:1 ratio of brominated imide to antimony., avail- able as PE Conc. 1Nat-2P-W from M.A Hannah, Elk Grove Village, IL. LLDPE 6806 Liner lowdensity polyethylene, available from Dow Chemical Co., Midland MI.SpaceRite ™ Alumina trihydroxide, available from Alcoa Chemicals, S11Charlotte, NC. Engage ™ A metallocene polymerized olefin, containing 24%8100 octane comonomer available from Dow Chemical Co., Midland, MI. LDPE1550 Low density polyethylene, available from Eastman Chemical Products,Inc., Kingsport, TN. ATH FR Alumina trihydroxide compounded withethylene vinyl acetate at a 60% by weight concentration, available fromMach 1 Compounding, Macedonia, Ohio. Elvax ™ 410 An ethylenevinyl-acetate copolymer available from E.I. DuPont de Nemours, Inc.,Wilmington DE. O521-48 FR Magnesium hydroxide compounded withpolypropylene at a 50% by weight concentration, available from Mach 1Compounding, Macedonia, Ohio. Escorene ™ Isotactic polypropyleneavailable from Exxon Chemical 3445 Co. Environstrand A blend oftetrabromobisphenol A with antimony oxide in atactic polypropylene,available as Envirostrand 5P280 from Great Lakes Chemical, WestLafayette, IN PPSC 912 An ethylene-propylene copolymer with a melt indexof 65, available as Profax SC 912 from Montell North America, WilmingtonDE

Examples 1-3, Comparative Examples 1-4

Examples 1-3 were multilayer films having 13 layers of a constructionA(BA)₅BA. They were prepared to illustrate the effect on overall flameretardant properties of using various amounts of a flame retardantadditive in a B layer compared to using similar amounts in a blendedcomposition.

In Example 1, the non flame retardant layers were made of Rexflex™ W101and 35% Fina™ 4 3374, melt mixed in a weight ratio of 65:35 and conveyedin a BERLYN single screw extruder (BERLYN, 51 mm, having an L/D of 30/1,commercially available from Berlyn Corp., Worchester, Mass., operatingwith zone temperatures increasing from 149° C. to 238° C.) to A slots ofa feedblock having 13 slots. The feedblock, made as described in U.S.Pat. No. 4,908,278 (Bland et al.), allowed two flow streams fed into the13 slots in an alternating manner to come together in a multilayer flowstream having layers arranged as A(BA)₅BA. The temperature of both thefeedblock and the die were set at 204° C. The flame retardant layerswere made from 1 Nat-2P-W, fed by a single screw extruder (KILLION ModelKTS-125, 32 mm, having an L/D of 24/1, commercially available fromKillion, Inc., Cedar Grove, N.J.) operating with zone temperaturesincreasing from 132° C. to 238° C. into B slots of the feedblock. Theresulting multilayered flow stream was passed through a single orificefilm die and drop cast onto a chill roll set at a temperature of 15° C.and collected. The line speed was 4.6 m/min., the individual flowratesof A and B were such that flame retardant material comprised acalculated 14.3 weight percent of the overall multilayered film and theoverall thickness was measured at 150 micrometers.

Examples 2 and 3 were made essentially as in Example 1, except the flowrates of the materials were adjusted to obtain flame retardantconcentrations of 33.3 and 47 weight percent, respectively.

In Comparative Example 1, the same material used in the A layer ofExample 1 was fed into the Berlyn extruder of Example 1, conveyedthrough a feedblock and a single layer die and drop cast onto a chillroll. The temperatures of the extruder increased from 149° C. to 238°C., the feedblock and the die were set at a temperature of 204° C. andthe chill roll was set at a temperature of 15° C. The overall thicknesswas 150 micrometers and the flame retardant concentration was 0 weightpercent.

Comparative Examples 2-4 were made essentially as in Comparative Example1 except the flame retardant additive used in the B layers of Example 1was melt blended to result in an overall flame retardant concentrationin weight percent of 14.3, 33.3 and 47, respectively.

Examples 1-3 and Comparative Examples 1-4 were tested for Hanging StripFlammability, and Horizontal Burn. The test results, film layers andflame retardant concentrations are shown in Tables 1 and 2.

TABLE 1 FR Hanging Strip Ex. Layers % Flame Comments  1 13 14.3 3 sec,SE Flaming drips  2 13 33.3 3 sec, SE Flaming drips, hard to ignite  313 47.0 <1 sec, SE  Very difficult to ignite C1  1 0.0  34 sec, 38 cmFlaming drips C2  1 14.3  17 sec, 38 cm Flaming drips, easy to ignite,C3  1 33.3 15 sec, SE  Flaming drips C4  1 47.0 2 sec, SE No drips

As seen, the films of the invention exhibited improved flame retardantperformance as blends having the same overall concentration of flameretardant material.

TABLE 2 FR Horizontal Burn Ex. Layers % Flame Comments  1 13 14.3 18 secNo drips, high char  2 13 33.3  1 sec No drips, high char  3 13 47.0 <1sec No drips, high char C1  1 0.0 127 sec  Flame drips, all tape burnedC2  1 14.3 11 sec Flame drips, low char C3  1 33.3 16 sec No drips, highchar C4  1 47.0  4 sec No drips, high char

As seen, the films of the invention exhibited improved flame retardantperformance over blends having the same overall concentration of flameretardant material when the flame-retardant concentration wassufficient.

Example 4 and Comparative Examples 5-6

Example 4 illustrates the effect of two flame retardant materials in theflame-retardant layer on overall flame retardant performance.

A multilayer film was made essentially as in Example 1, varying thepolymer matrix. Flame retardant additive and process conditions asnoted. The non flame retardant “A” layers were made of Reflex™ W101 andFina™ 3374 in a weight ratio of 75:25 instead of 65:35. The flameretardant “B” layers were made of a mixture of LLDPE 6806, 1-Nat-2P-Wand Alcoa Spacerite™ S11 in a weight ratio of 25:56:19. The settemperature in the Berlyn extruder varied from 138° C. up to 193° C. Thetemperature in the Killion extruder varied from 132° C. up to 182° C.Die and feedblock at 193°.

Comparative Example 5 was made essentially as in Comparative Example 1except the non flame retardant polymer was made of Reflex™ W101 andFina™ 3374 in a weight ratio of 75:25 instead of 65:35.

Comparative Example 6 was performed essentially as in ComparativeExample 2 except as follows. The non flame retardant material was madeof Reflex™ W101 and Fina™ 3374 in a weight ratio of 75:25 instead of65:35. The flame retardant mixture used in the “B” layer of Example 4was melt blended with the non flame retardant material to result in anoverall flame retardant concentration of weight percent of 35.

Examples 1-3 and Comparative Examples 1-4 were tested for Hanging StripFlammability and for Tensile Stress. The test results, film layers andflame retardant concentrations are shown in Table 3.

TABLE 3 Tensile Stress FR Hanging Strip At 10% strain Ex. Layers % FlameComments KPa (psi)  4 13  35  <1 sec, SE¹ Drips but not flaming 535(775)C5 1  0 27 sec, all Flaming drips 635(920) C6 1 35  4 sec, SE Flamingdrips 597(865) ¹Self extinguished immediately after burner removed,could not be ignited.

As seen, a film of the invention exhibited improved flame retardantperformance of a blend having the same overall concentration of flameretardant. The lower tensile stress value of Example 4 was attributed topoor mixing.

EXAMPLES 5-6

These examples were prepared to illustrate the use of halogen-free flameretardant materials with two different non-flame retardant materials.

Examples 5 and 6 were made in a manner similar to Example 1 except thematerials were different and some process conditions were changed. InExample 5, the materials used in the not flame retardant “A” layers wereEngage™ 8100 and LDPE 1550 melt blended in a weight ratio of 50:50. InExample 6, the materials used in the non-flame retardant “A” layers wereReflex™ W101 and Fina™ 3374 in a weight ratio of 75:25 instead of 65:35.In both examples, the materials used in the flame retardant “B” layerswere a mixture of ATH FR and Elvax™ 410 in a weight ratio of 90:10. AKILLION single screw extruder (KILLION Model KTS-125, 32 mm single screwextruder with L/D of 24/1 and fitted with a mixing screw containing anEagan mixing section) was used instead of a BERLYN single screw extruderto convey the non-flame retardant material to the “A” slots of thefeedblock and a second KILLION single screw extruder, also fitted with amixing screw containing an Eagan mixing section, conveyed the flameretardant material to the “B” slots. In examples 5 and 6, the firstKILLION extruder was operated at temperatures in the first zone to thelast zone ranging from 149° C. to 177° C. and 149° C. to 188° C.,respectively. In both examples the temperatures of the feedblock, dieand chill roll were maintained at 177° C., 177° C. and 20° C.,respectively. The film speeds for Examples 5 and 6 were 4.9 and 5.5m/min, respectively. The film thickness for both was about 130 microns.

Comparative Example 7 was made as in Example 5 except no flame retardantmaterial was fed into the “B” slots and the flowrate was adjusted toresult in a one layer film having a thickness of about 130 micrometerswhere all seven layers merged into a single indistinguishable layer.

Examples 5-6 and Comparative Example 8 were tested for Horizontal Burn.The test results, film layers and flame retardant concentrations areshown in Table 4.

TABLE 4 FR Horizontal Burn Ex. Layers % Flame Comments 5 13 37 13 secNo/low char, 2-3 drips, flames do not burn down length of rod, tape doesnot burn readily, bubbling during burning & no smoke. 6 13 38 16 secSame as Ex 5 except 1-2 drips. C7   1  0 120 sec  No char, flamingdrips, burns entire length of rod and light smoke.

As seen, the films of the invention exhibited substantial flameretardant performance.

EXAMPLE 7

These examples were prepared to illustrate the use of an inorganic flameretardant additive with two different non flame retardant materials.

Example 7 was made essentially as in Example 6 except as follows. Thematerials used in the non-flame retardant “A” layers were Rexflex™ W101and Fina™ 3374 in a weight ratio of 75:25. The flame retardant additivein the “B” layer was 0521-48. The temperatures for the “A” layerextruder and the “B” layer extruder were set to increase from between182° C. and 204° C. and between 171° C. and 227° C., respectively.

Comparative Example 8 was made essentially as in Example 7 except noflame retardant additive was fed into the “B” slots and the flow ratewas adjusted to result in a one layer film having a thickness of about130 micrometers.

Example 7 and Comparative Example 8 were tested for Horizontal Burn. Thetest results, film layers and flame retardant concentrations are shownin Table 5.

TABLE 5 FR Horizontal Burn Ex. Layers % Flame Comments  7 13 42  18 secHigh char, no drips, flames do not burn down length of rod, tape doesnot burn readily, flakes/ash produced during burning & no smoke. C8  1 0 127 sec Flaming drips and all tape burned.

As seen, the films of the invention exhibited substantial flameretardant performance.

EXAMPLES 8-10

These examples were prepared to illustrate the effect of flame retardantmaterials that melted a processing temperatures on layer thickness.

Example 8 was made essentially as in Example 1 except some equipment,processing conditions and materials were the different. The not flameretardant “A” layers were made of Escorene™ 3445 and the flame retardant“B” layers were made of a mixture of 50% Great Lakes Environstrand and50% PPSC 912. The “B” layer material was conveyed to the “B” slots witha twin screw extruder (LEISTRITZ Model LSM 34 GL, 34 mm, having 42/1,commercially available from Leistritz Corp., Sommerville, N.J.). Thetemperature of the extruder for the “A” layers varied from 160° C. up to193° C. The temperature of the extruder for the “B” layers ranged from150° C. up to 177° C. The individual flowrates of A and B were such thatflame retardant material comprised a calculated 25 weight percent of theoverall multilayered film and the overall thickness was measured at 100micrometers.

Examples 9 and 10 were made in a similar manner to Example 8 exceptExample 9 used a 29 layer feedblock and Example 10 used a 91 layerfeedblock.

Examples 8-10 were tested for both Hanging Strip and Horizontal Burn.The test results, film layers and flame retardant concentrations areshown in Table 6.

TABLE 6 Ex. Layers FR % Hanging Strip Horizontal Burn  8 13 25 Meltedand dripped <1 sec, low char, low smoke, would not ignite extinguishesupon removal of flame  9 29 25 Melted and dripped <1 sec, low char, lowsmoke, would not ignite extinguishes upon removal of flame 10 91 25Melted and dripped <1 sec, low char, low smoke, would not igniteextinguishes upon removal of flame

As seen, the thickness of the flame retardant layer could be quite thinwithout adversely affecting the flame-retardant performance of theoverall film by loss of layer integrity.

Each of the patents, patent applications, and publications cited hereinis incorporated by reference as if each were individually incorporatedby reference. The various modifications and alterations of thisinvention will be apparent to those skilled in the art without departingfrom the scope and spirit of this invention.

What is claimed is:
 1. A unified multilayer film of at least fivecontinuous film layers wherein at least one layer comprises a flameretardant film layer and at least one layer comprises a non-flameretardant film layer, wherein the flame retardant film comprises ahalogenated organic flame retardant additive incorporated into a meltprocessible polymer.
 2. The multilayer film of claim 1 wherein at leastone internal layer comprises a flame retardant film layer.
 3. Themultilayer film of claim 1 that comprises flame-retardant film layersalternating with non flame retardant film layers.
 4. The multilayer filmof claim 3 wherein the flame retardant of the flame retardant filmslayers comprises the same flame retardant.
 5. The multilayer film ofclaim 3 wherein the flame retardant of the flame retardant films layerscomprises different flame retardants.
 6. The multilayer film of claim 1wherein the flame retardant additive comprises a mixture of two or moredifferent flame retardant additives.
 7. The multilayer film of claim 1wherein the two outermost layers comprise a non flame retardant film. 8.The multilayer film of claim 1 which is oriented.
 9. The multilayer filmof claim 1 comprising a flame retardant film layer, a non flameretardant film layer, and a tie layer therebetween.
 10. The multilayerfilm of claim 1 comprising at least ten layers.
 11. The multilayer filmof claim 1 further comprising at least one pressure sensitive adhesivelayer on at least one surface.
 12. The multilayer film of claim 1wherein the flame retardant additive comprises from 10 to 90 weightpercent of the unified multilayer film.
 13. The multilayer film of claim1 wherein the flame retardant additive comprises 40 wt. % or more ineach flame retardant layer.
 14. The multilayer article of claim 1,wherein said melt processible polymer is a thermoplastic, athermoplastic elastomeric or elastomeric material.
 15. The multilayerarticle of claim 14, wherein said melt processible polymer is selectedfrom the group consisting of homo- and copolymers of ethylene, propyleneand butylenes.
 16. The multilayer article of claim 14, wherein said athermoplastic elastomeric material is selected from the group consistingof styrene-isoprene block copolymers, styrene-(ethylene-butylene) blockcopolymers, styrene-(ethylene-propylene) block copolymers,styrene-butadiene block copolymers; polyetheresters; elastomericethylene-propylene copolymers; thermoplastic elastomeric polyurethanes;polyvinylethers; and poly-α-olefins.
 17. The multilayer article of claim1 wherein said flame retardant is an antimony-halogen additive.